Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. In a typical leak test, a vacuum is applied to the fuel system. The integrity of the system is determined by monitoring the decay of the applied vacuum or by comparing the resulting fuel system pressure to an expected pressure. The vacuum source may be the intake manifold of the vehicle engine.
In some vehicles, such as hybrid electric vehicles, the vehicle engine may not run frequently, or may not generate enough vacuum to conduct a leak test. Such vehicles are required to have an evaporative leak check module (ELCM) coupled to the fuel system. The ELCM includes a vacuum pump that can be coupled to the fuel system for leak testing. When applying a vacuum to the fuel tank, fuel vapors may be drawn into the fuel vapor canister. Again, the limited engine run time may limit opportunities to purge the fuel vapor canister to intake. In particular, if the leak check is performed following a key-off event, the loaded fuel vapor canister may soak over a diurnal cycle, increasing bleed emissions.
One example approach, shown in U.S. Pat. No. 8,074,627, includes removing fuel from a sealed fuel tank to an accumulator, thereby decreasing the volume of fuel in the fuel tank and decreasing the vapor pressure there within. However, this requires operation of the fuel pump, adding additional wear to an essential component of vehicle operation. Further, this adds additional components and complexity to the fuel delivery system, increasing both manufacturing costs and the likelihood of a malfunction within the system.
The inventors herein have recognized the above problems, and have developed systems and methods to at least partially address them. In one example, a method for a fuel system is provided, comprising adjusting a volume of a sealed fuel tank while maintaining spatial compartmentalization of bulk fuel within the sealed fuel tank, and indicating degradation of the sealed fuel tank based on a change in fuel tank pressure. In this way, the fuel tank may be tested for leaks without being unsealed, and without engaging the fuel pump or other elements of the fuel delivery system.
In another example, a fuel system for a vehicle is provided, comprising a fuel tank sealable via closing a fuel tank isolation valve coupled between the fuel tank and a fuel vapor canister, and a diaphragm valve coupled between the fuel tank and the fuel vapor canister. In this way, the fuel tank pressure may be adjusted via the diaphragm valve, and without venting fuel vapor to the fuel vapor canister during leak testing, thereby eliminating potential bleed emissions.
In yet another example, a method for a fuel system, comprising applying a vacuum to a fuel vapor canister side of the fuel system while maintaining a fuel tank isolation valve closed, and indicating degradation of a fuel tank based on a change of fuel tank pressure. In this way, the fuel vapor canister will not be loaded during leak testing, enabling the installation of smaller canisters.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.